This invention relates to an eddy current probe and particularly to a probe for accurately measuring a gap between a tube and an antivibration bar in a steam generator for commercial nuclear power plants.
The design and operation of nuclear power plants is well known in the art. For example, U.S. patent application No. 729,385, filed May 1, 1985, now U.S. Pat. No. 4,653,576, by H. O. Lagally, and assigned to the Westinghouse Electric Corporation, the contents of which are hereby incorporated by reference, describes the structure and operation of a nuclear steam generator. As described therein, a nuclear steam generator includes a large number of U-shaped tubes oriented along the longitudinal axis of a cylinder. A reactor coolant flows into exposed openings of the plurality of U-shaped tubes and feedwater which passes up around the outside of the tubes, absorbs heat from the reactor coolant flowing within the tubes. The heat absorbed causes the feedwater to boil and produce steam which is used to drive an electrical generator.
The U-shaped tubes are arranged in columns and supported in a bundle, and a plurality of antivibration bars are typically disposed between each column of the U-shaped tubes. The configuration of the tubes is such that there is up to 10 feet between the legs of each U-shaped tube and the U-shaped tubes have a height of up to 35 feet. In the tube bundle, the tubes are arranged with a gap of approximately 1/2 inch between the tubes.
The antivibration bars each comprise a bar which is bent into a V-shaped configuration, so that two legs are formed with an angle therebetween. The V ends of the bars are inserted between the U-shaped flow tubes and the free ends of the V-shaped antivibration bars are welded to opposite sides of a corresponding retainer ring. Thus, the antivibration bars are used to provide support along the length of the curved or U-shaped portion of the flow tubes. The antivibration bars are intended to prevent vibrations of the individual tubes in the tube bundle. It is known that such vibrations are caused by the flow of water and steam past the flow tubes, and that the U-shaped portion of the tube bundle is most severely affected by the vibrations. The flow-induced vibrations can cause damage to the flow tubes because of wear.
The U-shaped tubes of the tube bundle have dimensional tolerances associated with their outer diameter. There are also variations caused by ovalization of the tubes as a result of the bending of the tube in the U-shaped portion. Furthermore, the spatial relationship between adjacent tubes varies, although it is within set design limits. Thus, there is a dimensional tolerance associated with the nominal spacing between the steam generator tubes. There is also a dimensional tolerance associated with the outer dimensions of the antivibration bars which typically comprise round tubes. The antivibration bars may also comprise a square, oval, or any other shape having a uniform or non-uniform cross-section. The combination of these tolerances and dimensional variances prevents the elimination of gaps between the antivibration bars and the tubes of the steam generator. Such gaps are undesirable because they allow vibration of the tubes and relative motion between the tubes and the antivibration bars. This relative motion can cause wear and subsequent failure of the tubes of the steam generator. If the gap between the tube and the antivibration bar is small, then any vibration of the tube as it moves to hit the bar will be limited. However, if the gap becomes larger, the tube will gain additional energy as it vibrates and contacts the antivibration bar, so that wear or fretting of the tube will result.
In order to avoid the vibration and wear problems which can be caused by a large gap between a tube and an antivibration bar, a proposed solution in newly designed steam generators is to limit the clearance between the tube and the antivibration bar. In order to verify that the antivibration bar is correctly located with respect to the adjacent tube, measurement of the spacing between the tube and the bar must be accomplished.
One apparatus which has been used to measure the spacing between a tube and an antivibration bar is a "feeler" gauge mounted on top of a rod. The feeler gauge is inserted between the bar and the tube. However, because of the relatively large size of the U-shaped tubes making up the tube bundle, and the relatively small thickness of the feeler gauge (approximately 0.005 inches), remote measurement of the gap is difficult.
Eddy current inspection is routinely performed as part of an in-service inspection of a nuclear steam generator to provide data that can be used to locate support structures of a machine. Measurements used in conventional eddy current probes have been shown to be correlated to the dimensions of the structure, so that it has been proposed that an eddy current inspection could be used to measure the gap between an antivibration bar and a tube. However, for each tube there are two adjacent antivibration bars on opposite sides of the tube. Since conventional eddy current probes sense the entire circumference of the tube, a conventional eddy current probe will measure the sum of the gaps between the tube and both of the adjacent antivibration bars. Thus, a gap for an individual antivibration bar cannot be measured, so that there is a need for a probe which inspects only a portion of the tube circumference.
While 8 coil probes are available for sensing limited portions of the tube circumference, these coils are difficult to position properly to accurately sense the position of an antivibration bar. Such probes are designed with 8 coils on the circumference, with each coil sensing only a limited portion of the tube circumference. Since an antivibration bar and the tube are directly adjacent one another at only one location on the circumference of the tube, a coil with a limited view may not be positioned properly to accurately sense the gap between the bar and the tube. Experimental measurements performed with existing multiple coil probes confirmed that there is a strong dependence of the gap measurement capability on the angular position of the antivibration bar with respect to the individual sensing coil. In view of the 35 foot length of the tube, it is also difficult to control the orientation of a probe so as to achieve an accurate measurement. Thus, there remains a need in the art for a probe which is capable of accurately measuring the gap between a tube and a single adjacent antivibration bar. There is also a need for such a probe which can be readily oriented in the U-shaped portion of a flow tube despite the relatively long length of the flow tube.